1. Field of the Invention
This invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the invention relates to both primary and secondary non-aqueous electrolyte lithium-containing electrochemical cells. Such cells are typically used to power implantable medical devices, for example cardiac defibrillators. In particular, the invention relates to a quaternary solvent system including a cyclic carbonate, linear di-ether, linear carbonate and linear mono-ether. The solvent system provides an electrolyte having higher conductivity than conventional solvent electrolytes.
2. Prior Art
The successful production of lithium electrochemical cells and their widespread application are largely dependent on the development of highly conductive and stable non-aqueous organic electrolytes. Non-aqueous organic electrolytes are composed of a salt dissolved in an organic solvent system of either a single solvent or mixed solvent. A general requirement of non-aqueous organic electrolytes is that they be reductively and oxidatively stable towards both anode active materials, for example, lithium metal and lithiated carbon, and typically used cathode active materials, for example, silver vanadium oxide (SVO), copper silver vanadium oxide (CSVO), fluorinated carbon (CFx), manganese oxide (MnO2), cobalt oxide (CoO2), and others. For a high rate lithium cell application, an activating electrolyte with high conductivity is especially significant. To achieve high electrolyte conductivity, a combination of two solvents, one with a high dielectric constant and one with a low viscosity, is generally used.
Many lithium salts and organic solvents have been successfully used in lithium electrochemical cells including LiAsF6, LiPF6, LiBF4, LiClO4, LiSO3CF3, among others. Typically used solvents include propylene carbonate (PC), ethylene carbonate (EC), γ-butyrolactone (GBL), sulfolane, 1,2-dimethoxyethane (DME), dimethyl carbonate (DMC), tetrahydrofuran (THF), diisopropyl ether (DIPE) 1,3-dioxolane, and others. One particularly stable and highly conductive electrolyte is 1.0 M LiAsF6 or LiPF6 in PC:DME=1:1. This electrolyte is widely used in the battery industry. A particularly common application is in a high rate Li/SVO defibrillator cell using LiAsF6 as the preferred electrolyte salt.
Despite the success of 1.0 M LiAsF6/PC:DME=1:1 electrolyte, a better electrolyte with higher conductivity and stability is needed in the present investigation for high rate, high power, and high capacity electrochemical cells. It is interesting to note that the above-discussed electrolyte using a PC/DME solvent system does not provide maximum conductivity at its one-to-one volume ratio. As shown in FIG. 1, the maximum conductivity of 1.0 M LiAsF6 in PC/DME is at a volume ratio of 20:80. The conductivity of DME of about 19.5 mmho/cm at 37° C. is about 12.4% higher than that of the PC at 17.3 mmho/cm at 37° C.
The benefit of using an electrolyte of 1.0 M LiAsF6 dissolved in a solvent system of PC:DME at a volume ratio less than 5:5, i.e., down to 2:8, which has a higher conductivity, seems obvious for high rate lithium electrochemical cells. However, an acceptable electrolyte must provide both high conductivity and high stability toward both the cathode and the anode. The first requirement of a good electrolyte is to significantly reduce or minimize the internal resistance (IR) voltage drop during high current pulse discharge. The second requirement is to minimize the impedance build-up at the solid electrolyte interface (SEI) at the anode and the cathode. Therefore, high electrolyte conductivity does not necessarily mean better cell performance or improved discharge capacity. Indeed, when electrolytes of 1.0 M LiAsF6/PC:DME=4:6 or 3:7 are used in Li/SVO cells, the benefit of their high conductivity in a short term discharge test is completely canceled by the presence of larger voltage delay during high current pulse discharge applications.
It is believed that voltage delay in Li/SVO cells is caused by the dissolution of vanadium ions from the cathode into the electrolyte, which then re-deposit on the anode surface by reduction to produce a highly resistant surface film. The ion dissolution process is catalyzed by the presence of DME, which is a very good ligand molecule. This linear ether has a larger donation number (DN=20) than the does propylene carbonate having DN=15.1. The donation number signifies the potential of a nucleophile molecule to donate an electron pair as described in the Lewis acid-base theory. To minimize or even eliminate the voltage delay phenomenon, a lower percentage of DME in the electrolyte solvent mixture is desired. This decreases the content of DME with a high DN. However, by reducing the percentage of DME, the electrolyte conductivity is also decreased. Therefore, the electrolyte of 1.0 M LiAsF6/PC:DME=1:1 typically used to activate Li/SVO cells is a balanced choice of maximizing the solvent system conductivity and contemporaneously minimizing the undesirable effect of dissoluted vanadium ions in the electrolyte.
Although the 1.0 M LiAsF6/PC:DME=1:1 satisfies the present requirements in defibrillator cell applications, it slowly decomposes to form a relatively highly resistive surface film on the Li/SVO cell electrodes at certain discharge values, as signified by the voltage delay phenomena. For longer-term cell storage or usage, this phenomenon becomes more obvious and severe.
The invention of CSVO as a new cathode active material is important in the pursuit of the next generation high energy density and high power electrochemical cells. Copper silver vanadium oxide provides about 7% to 15% more capacity per gram than conventionally SVO cathode materials. This cathode material is described in U.S. Pat. Nos. 5,670,276 and 5,516,340, both to Takeuchi et al. These patents are assigned to the assignee of the current invention and incorporated herein by reference. In order to fully realize the improved capacity benefits of CSVO, however, a new electrolyte system that is more conductive and more stable toward both SVO and CSVO active cathode materials is needed.
One recently developed electrolyte system includes the ternary solvents of PC (cyclic carbonate), DME (linear di-ether) and DMC (linear carbonate). The latter compound has a dissociation number of about 15. Electrolytes made with this solvent system have significantly higher conductivities than that of the standard binary solvent (PC:DME=1.1) electrolyte, while functioning fairly well under discharge conditions to which Li/SVO cells are typically subjected. However, PC:DME:DMC electrolyte systems have shown instability under certain experimental conditions. Thus, although this ternary solvent electrolyte is commonly used to activate Li/SVO cells, there are some applications for which it is not useful.
To remedy the instability problem of PC:DME:DMC, another ternary solvent electrolyte system was developed and is described by U.S. Pat. No. 5,776,635 to Gan et al. This patent is assigned to the assignee of the current invention and incorporated herein by reference. The solvent system includes PC, DME, and (DIPE). Diisopropyl ether has a dissociation number of less than 19. Both Li/SVO and Li/CSVO cells activated with this electrolyte system exhibit very good chemical and electrochemical stability in comparison to electrolytes of PC:DME as well as the newer PC:DME:DMC electrolyte systems. Even though this electrolyte system is advantageous in terms of its long-term performance and stability in Li/SVO and Li/CSVO primary cells, its conductivity is only comparable to that of the standard binary solvent electrolyte of PC:DME. Thus, an electrolyte system is needed that is stable chemically and electrochemically while having a higher conductivity than the conventional binary solvent electrolyte.
Accordingly, the present invention is directed to an electrolyte system that is more conductive than the conventional binary solvent electrolyte while being chemically and electrochemically stable toward Li/SVO and Li/CSVO primary electrochemical systems as well as secondary lithium ion chemistries.